Low idle/stall in warm weather

So, I have a 97 f150 with a 4.6 that idles low, around 5-600 all the time, and stalls in warm weather only. I just cleaned the IAC to no avail, and am looking for any more tips or fixes for this problem.

When the engine is up to temperature and idling, take the handle of a srewdriver and tap on the IAC valve. If the engine idle speed changes replace the IAC valve.


As Tester says, it’s probably a stubborn IAC. But there’s a chance it is a problem with the coolant temp sensor too. If you test it, make sure you are testing the sensor that the ECM uses, not the one for the dash guage.

Since it is stalling in warm weather only, it could be that the mixture is too rich. Have you had the ECM codes read? Is there any other indication of too-rich operation? Poor gas mileage, spark plugs fouling, etc.?

Poor gas mileage. Yeah, that could probably be it, since I’m getting an average of 8 mpg right now, at best. Haven’t had ECM codes checked anytime recently, but I should be able to take it into school, they do it for free. Hah.

Another weird thing. Sometimes, on a cold start, when the engine catches, it’ll start roaring to like, catch and hold around 700, then run normal cold idle, 12-15ish after a second.

Edit: to life, not like. Durr.

@antmanzero sounds like a faulty IAC valve. FWIW . . . cleaning them doesn’t always work. Sometimes they’re just faulty.

I take it you’ve already tried cleaning the throttle body?

I agree with @GeorgeSanJose . . . consider changing that ECT sensor.

Also consider changing the thermostat, also because of the poor fuel economy.

And please let us know what fixes your problems.

sounds like maybe a stuck open or leaking egr valve. check your engine’s vacuum reading at idle. if it’s under around 18", that’s a likely cause. use a business between the valve and it’s mounting surface to block any flow. if that fixxes it, replace the valve.

Rough idle/low power due to spark plug cap popping off number 7. It keeps popping off and really confusing me. I let my school auto shop have at it, and they pulled a code for a faulty IAC motor.

Final comment on the matter: Replaced IAC, solved stalling, low idle, and some power issues. Crimped the end of the one spark plug wire, and now I’m running just about perfectly.



Unfortunately, IAC valves commonly fail on Fords